ctdb-tests: Continue running if a testcase is not executable

ctdb-tests: Continue running if a testcase is not executable

At the moment the whole test run aborts without printing a summary of
results but inexplicably succeeds.  Instead, generate a clear failure
for a non-executable testcase.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>